The present invention relates generally to the field of transitions to and from waveguides and, more specifically, to the field of transitions from waveguide to balanced transmission lines and still more specifically, to spiral balanced transmission lines.
A printed circuit spiral antenna is a balanced transmission line wound into a spiral. Spiral antennas are very desirable for use in electronic warfare systems because such antennas have very broadband and broadbeam characteristics. The operable frequency range of a single antenna is as high as 2-18 GHz. The beamwidth of these antennas is about 90% and a set of four such antennas are commonly used for determining a signal's angles of arrival by means of amplitude comparison circuits. The balanced transmission lines that form such spiral antennas are normally matched to the commonly used coaxial transmission line by means of a balun. At higher frequencies, i.e. above 18 GHz, the size of the coaxial line must be very small to avoid moding and, hence, the balun becomes very difficult to assemble. The performance of spiral antennas at the millimeter wave frequencies is also degraded by the loss and VSWR of coaxial baluns. In general, coaxial lines are not desired for millimeter wave circuits. Thus, spiral antennas are better matched for waveguide circuits such as mixers and switches. It has been found that the optimum assembly of a millimeter wave circuit is achieved by integration of the various circuit components. Hence, it is desirable to provide a transition from the antenna that can be integrated with the normal millimeter wave circuits and hence utilize a balanced transmission line suitable for waveguide circuits.